A Long-Term Evolution (LTE) system offers high peak data rates, low latency, improved system capacity, and low operating cost resulting from simple network architecture. An LTE system also provides seamless integration to older wireless network, such as GSM, CDMA and Universal Mobile Telecommunication System (UMTS). In LTE systems, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNodeBs or eNBs) communicating with a plurality of mobile stations, referred as user equipments (UEs). Enhancements to LTE systems are considered so that they can meet or exceed International Mobile Telecommunications Advanced (IMT-Advanced) fourth generation (4G) standard.
One of the key enhancements is to support bandwidth up to 100 MHz and be backwards compatible with the existing wireless network system. Carrier aggregation (CA) is introduced to improve the system throughput. With carrier aggregation, the LTE-Advance system can support peak target data rates in excess of 1 Gbps in the downlink (DL) and 500 Mbps in the uplink (UL). Such technology is attractive because it allows operators to aggregate several smaller contiguous or non-continuous component carriers (CC) to provide a larger system bandwidth, and provides backward compatibility by allowing legacy users to access the system by using one of the CCs.
A technique referred to as Hybrid Automatic Repeat ReQuest (HARQ) is employed for error detection and correction. In a standard Automatic Repeat ReQuest (ARQ) method, error detection bits are added to data to be transmitted. In Hybrid ARQ, error correction bits are also added. When the receiver receives a data transmission, the receiver uses the error detection bits to determine if data has been lost. If it has, then the receiver may be able to use the error correction bits to recover (decode) the lost data. If the receiver is not able to recover the lost data using the error correction bits, then the receiver may use a second transmission of additional data (including more error correction information) to recover the data. Error correction can be performed by combining information from the initial transmission with additional information from one or more subsequent retransmissions.
In order to perform HARQ, certain amount of HARQ memory space is required to store the data. Furthermore, multiple HARQ processes are needed to detect and recover multiple erroneous transport blocks (TBs). For LTE systems in FDD, the HARQ memory space is equally divided for 8 HARQ processes for transmission modes (TMs) without spatial multiplexing, and 16 HARQ processes for TMs with spatial multiplexing. For LTE systems in TDD, the number of required HARQ processes varies based on the corresponding TDD UL-DL configuration. The buffer size for each HARQ process is calculated by considering the maximum TB size (TBS). Depending on UE category and TM, either full buffer size or limited buffer rate matching (LBRM) is designed for each HARQ process. Already in Rel-8/9, UE may be short of HARQ processes since the specified minimum requirement on the number of HARQ processes can be smaller than the maximally required number. This may have an impact on the peak system throughput if conservative scheduling method is used by the network.
With carrier aggregation, the HARQ memory space is not increased to take into account CA behavior for UE category 1-5 due to backwards compatibility. The buffer size for each HARQ process is still equally divided among serving cells in CA scenarios, which makes the received coded bits possible to be bigger than its buffer size. For UE category 6-7, although the total soft channel bit buffer size is increased, it is designed to be backwards compatible to UE category four. All of these indicates that the soft channel bit buffer may be smaller than what is assumed at eNB. In LTE Rel-10, rules are defined for discarding overflowed coded bits at UE side, while the rate matching at eNB remains the same. This creates a mismatch between eNB rate matching and UE soft bit storing for both FDD and TDD modes and performance degradation is expected. In LTE Rel-11, different TDD UL-DL configuration among aggregated cells are supported. As a result, the maximum number of required HARQ processes may not be the same, thus different HARQ performance among different serving cells is expected. A solution is sought to provide intelligent HARQ buffer management for reducing the mismatch between rate matching at eNB and soft buffer storing at UE when the total number of active HARQ processes is small, and for increasing the total number of HARQ processes than minimum requirements when TBS is small.